In many laser applications involving single mode lasers and optical fibers, it is desirable to launch a beam of laser light having a stable frequency into an optical fiber. To perform this operation, it is necessary to focus the laser beam into the fiber core. A simplified cross-sectional view of a conventional system for accomplishing this operation is shown in FIG. 1. Laser 1 includes a laser cavity of length L.sub.1 spanned by partially reflecting mirrors 4 and 5. Light transmitted through mirror 5 is focussed by lens 2 into the core of optical fiber 3.
However, conventional systems such as that shown in FIG. 1 have caused the laser frequency to drift out of control due to reflection of the laser beam from the optical fiber back into the laser cavity. This reflected laser light (represented by light waves B) re-enters the laser cavity along the same path as the incident light (represented by light waves A) exits the laser cavity, so that laser light will tend to propagate as a standing wave in the effective "cavity" between the laser mirror farthest from the fiber (mirror 4) and the end surface of the fiber. Because the length, L.sub.2, of such effective cavity exceeds the length, L.sub.1, of the actual laser cavity spanned by the laser mirrors, the laser stabilization control system will respond as if the length of the actual cavity had increased. The stabilization control system will thus attempt to compensate for the apparent cavity length increase, causing the laser frequency to drift out of control. For many applications, such as where the laser light is used in an interferometer system, extremely small amounts of frequency drift may be unacceptable. In a typical conventional system, if as little as less than 0.1% of the laser output re-enters the laser cavity as a result of such a back reflection, unacceptable frequency drift results. Because such a small amount of back reflection may cause undesirable frequency drift, conventional laser isolators and highly efficient conventional antireflection coatings on the optical components between the laser cavity and the optical fiber are unsuitable for reducing such frequency drift to acceptable levels.